2019 Volume 60 Issue 3 Pages 450-457
Tensile strength and work hardening characteristics in the case that strain was applied from multi-axial directions by press processing and in the case that uniaxial strain was applied by tensile test were compared. Furthermore, in a unified manner with the results of the tensile tests, fatigue tests on plate material subjected to multiaxial strain were conducted, and the strength reliability of the pressed product was evaluated. Tensile strength of mild-steel plate subjected to uniaxial or multiaxial plastic strain was increased. If a tensile test is carried out again after a certain period of time has elapsed by applying strain, not only yield stress but also tensile strength are increased. This trend is due to strain aging inside the material. Degree of work hardening is higher when multiaxial strain is applied compared to when uniaxial strain is applied. For materials work hardened by applying strain, their hardness and the tensile strength are proportional regardless of the load status of strain. Accordingly, tensile strength of a product subjected to different strains can be evaluated uniformly by measuring the product’s hardness. When strain is applied from multiple axes in press working, as well as tensile strength, fatigue strength is improved by work hardening. According to the results the microscopic analysis by X-ray diffraction combined with the full-width-at-half-maximum method, dislocation density is increased when multiaxial strain is applied compared to when single-axis strain is applied, and the degree of work hardening is greater. The full width at half maximum is correlated with tensile strength regardless of the state of strain applied to the material. TEM observation of dislocations revealed that the third-step-pressed product subjected to multiaxial strain has a coaxial, fine-grain dislocation-cell structure. On the other hand, in the case of pre-strained (plastic strain of 20%) material subjected to a uniaxial tensile load, the cell structure was unclear, and even the cells that were produced had coarse grains stretched in the tensile direction.
In regard to the manufacture of automotive parts, press (plastic) working is indispensable, and among the available processing methods, press forming of steel sheets produces press-worked products with a large specific gravity. Moreover, various steel plates with different microstructures and mechanical properties—ranging from mild steel plate to 1180-MPa-class high-strength tensile steel plate—are used for press-forming steel sheets. Many of these press-formed parts are given plastic strain by the press working, and the hardness and microstructure of the processed material are changed by this plastic strain. However, in the case of the current automobile design process, the strength of a designed part is set on the basis of the tensile strength of the steel sheet before pressing.
If a steel plate is subjected to a certain amount of plastic deformation in an unloading and reloading manner, its yield stress is increased by work hardening (which is caused by increased dislocation density in the steel sheet). In addition, it is known that the hardness of a material is improved by strain applied by plastic working, and an example in which the hardness of a mild-steel plate is nearly doubled by plastic working the plate has been reported.1) As reported, hardness corresponds to tensile strength,2–4) so tensile strength can also be improved by plastic deformation. When a steel sheet is subjected to uniaxial tensile plastic strain, its yield stress is increased, but its tensile strength is assumed to stay constant. On the contrary, it is known that rolled steel sheet has not only increased yield stress and hardness but also increased tensile strength and fatigue strength compared to those properties of the sheet before rolling.5,6)
It can therefore be considered that when plastic strain is applied from multiple axial directions, as in the manner of press forming, the change in mechanical properties of the strained material will be larger than that in the case of uniaxial tension. Car parts are molded from steel sheets by various press-working processes, and clarifying the influence of the plastic strain received in those processes on the static strength (yield stress, tensile strength, etc.) of the parts is a key task in regard to correctly evaluating the strength of the press-worked parts. Although many researches on tensile strength7–13) and fatigue strength14–20) of steel plates loaded with plastic strain have been reported, the majority of those reports deal with strain loads in the form of simple tensile deformation.5–20) Regarding strain load from multiaxial directions, although some examples of investigations on the strength characteristics of steel plate after rolling have been reported,5,6) few examples of directly investigating the strength characteristics of pressed products have been reported.
In consideration of the above background, in this study, focusing on the difference in strain-loading methods for press forming, we compared the influences on strength reliability of (i) strain in one direction (“uniaxial” strain hereafter) under simple tension (plastic strain of 5 to 20%) and (ii) strain from multiple axial directions (“multiaxial” strain hereafter). In particular, car parts made of mild-steel plate and manufactured by 10-step press working were targeted. Tensile specimens were cut out from blank material, molded into pressed products by one of three different pressing processes, and subjected to a tensile test. The effect of the plastic strain in the multiaxial direction received by the material in each process on the mechanical properties of the specimens was investigated. In addition to the mechanical properties determined by the tensile tests, strength reliability of the pressed products was comprehensively evaluated by cutting out test specimens from the pressed products and investigating their fatigue characteristics. The difference between the multiaxial strain load and the uniaxial strain load was investigated by observing the respective microscopic dislocation structures by TEM.
The mechanical properties of the test material (JSH 270 D) are listed in Table 1. For the pressed parts to be investigated, the first- to third-step pressed products of CVT pulley piston,1) which was processed from blank material with a diameter of 195 mm in 10 steps, were used. Photographs of the processed (press-worked) items after the first to third steps and their cross-sectional shapes are shown in Fig. 1. The first step is optimal cylindrical drawing with minimum thickness reduction. Specifically, minimum reduction in plate thickness is achieved by setting optimum punch radius R p, die radius R d (as shown in Fig. 2) and minimum blank-holding pressure. The second step is reverse-press forming in which the first-step-processed item is “reverse pressed” in order to greatly improve re-draw ability. Specifically, as shown in the schematic diagram of the forming process shown in of Fig. 2, the front and back surfaces of the steel sheet are inverted and processed by utilizing the rigidity of the pressed product itself. By this reversed forming of the front and back surfaces, stress in the direction opposite to that acted in the first step can be applied to the work-piece. As a result of this reverse press forming, which utilizes the so-called Bauschinger effect, formability is remarkably improved compared with that in the case of redrawing that is usually used.
Press worked products and cross section.
Schematic illustration of 2nd and 3rd press working process.
The third step involves intrusion forming to increase the wall thickness of the second-step-molded item to more than the material wall thickness. Specifically, as shown in the schematic diagram of the forming process in Fig. 2, the diameter of the vertical wall portion is reduced by pushing the second-step-molded product into the mold, and the wall thickness is increased according to the constant-volume law.
Measured wall thicknesses of the first- to third-step-pressed products are shown in Fig. 3. Tensile test specimens were prepared by cutting them out from vertical wall portions of these products by wire cutter. And these specimens were flattened in a vice and treated by machine before tensile test. To confirm the influence of the anisotropy of the steel sheet, as shown in Fig. 4, the test specimens was cut in three directions, namely, 0° (rolling direction), 45°, and 90° (right-angle), to the rolling direction of the steel sheet. The cutout positions and dimensions of the tensile-test specimens are shown in Fig. 5 and Fig. 6, respectively. These cutting positions were determined vertical wall portion due to small influence for the shape by press working. And the distance of vertical wall portions from the center of products on the first- to third-step-pressed products were difference. Therefore only maximum distance of 1st process were determined as shown in Fig. 5.
Thickness distribution of press products.
Angle for rolling direction.
Cutting location of specimen on tensile test.
Geometry of specimen on tensile test.
Tensile-test specimens, as shown in Fig. 7 were cut out from blank steel plate and subjected to three tensile strains (pre-strains) of 5%, 10%, and 20% as loads for uniaxial pre-strain. After tensile loading, a tensile specimen with the same shape as that made from the pressed product (Fig. 6) was cut out from the parallel part of the test specimen by wire cutting, and the tensile test was performed again.
Specimen for uniaxial tensile pre-straining.
For fatigue tests under multiaxial strain, test specimens were cut out from the press-molded products and blank materials by wire cutting in the same manner as the tensile specimens described above. Regarding the direction at an angle to the rolling direction, it was taken as the 45° direction only. The cutting positions of the fatigue-test specimens and the specimen dimensions are shown in Fig. 8 and Fig. 9, respectively. These test specimens were used for a tensile-tensile fatigue tests.
Cutting location of specimen on fatigue test.
Geometry of specimen on fatigue test.
For the uniaxial tensile pre-straining and tensile tests, an INSTRON universal testing machine (maximum load: 100 kN) was used.
Conditions for the tensile test were as follows. Tensile speed was 2 mm/min, Elongation was total elongation of the specimen. Number of specimen n = 1. Also, a Vickers hardness tester (measuring load: 200 gf) was used for measuring hardness of the test material. Furthermore, X-ray-diffraction peak profiles of the central part of the test specimens subjected to multiaxial and uniaxial strain were obtained by X-ray diffractometer, and the dislocation density (i.e., state of work hardening) in each specimen was investigated. Conditions for the X-ray-diffraction were as follows: Tube globe was used Cr material, Tube voltage was 40 kV, Tube current was 30 mA, Diameter of the collimater was 1 mm and 211 face was investigated. For the TEM, pick samples in the central part of specimen, and observed with 200 kV voltage.
For the fatigue test, a hydraulic fatigue tester (load control/dynamic maximum load: 80 kN) was used for fatigue tests with stress ratio R = 0.1.
The results of the tensile tests on the specimens cut at three angles to the rolling direction are plotted in Fig. 10(a) to (c), respectively. According to these results, tensile strength is improved as the pressing process progresses regardless of the specimen-cutting angle to the rolling direction. The results of the tensile tests on the test specimens (each direction) during each of the three process steps are plotted in Fig. 11(a) to (c), respectively. According to these results, no anisotropy is observed in each process, and the cut direction (i.e., sheet rolling direction) of the specimens does not significantly improve tensile strength.
Results of tensile test on each direction. (a) 0° direction, (b) 45° direction, (c) 90° direction.
Results of tensile on each process. (a) 1st process, (b) 2nd process, (c) 3rd process.
The above tensile-test results confirm that the tensile strength of the pressed product, namely, a mild-steel plate, was improved to approximately the same level as that of 590-MPa-class high-tensile-strength steel sheet after the third-step pressing was completed. In other words, this study verified that tensile strength is improved by work hardening in pressed products subjected to multiaxial strain.
The results of the tensile tests in the case that uniaxial pre-strain was applied are shown in Fig. 12. Clearly, as the amount pre-strain increases, both yield stress and tensile strength are increased. This result reveals that the stress-strain diagram obtained by the tensile test repeated after the pre-straining (“re-tensile strength” hereafter) does not coincide with the stress-strain diagram for the virgin mild-steel-sheet material considered up until now (namely, yield stress was improved, but tensile strength stays the same). The relationship between pre-strain amount and tensile strength is shown in Fig. 13. The broken line and dotted line show the tensile strength after each pressing step (as shown in Fig. 10(a)). As a result, although tensile strength is improved, even when the specimen is subjected to uniaxial strain (as shown in Fig. 12), it can be seen that the rate of improvement in tensile strength is lower than that in the case of applying multiaxial strain (as shown in Fig. 12 and Fig. 13).
Tensile test results of uniaxial pre-straining specimen.
Comparison of tensile strength between press worked products and uniaxial pre-straining specimen.
The results of a tensile test immediately after loading the test specimens with uniaxial pre-strain with elongation amount of 5 mm and those of a tensile test after a pre-strain load was applied for one month are shown in Fig. 14(a) and (b), respectively. This result indicated that the tensile test performed immediately after pre-strain loading produces equivalent results to a stress-strain diagram obtained by a continuous tensile test. On the contrary, when the tensile test is carried out after a lapse of time (one month), tensile strength is improved. This improvement in strength with the passage of time is due to “strain aging.” Strain aging is a phenomenon that occurs due to progress of sticking of dislocations with time as a result of stress fields generated in the material lattice by plastic strain.
Results of uniaxial tensile test with and without strain aging. (a) without strain aging, (b) with strain aging.
Conventionally, it is considered that in a tensile test on a uniaxial pre-strained material, yield stress improves but tensile strength does not change if the tensile test is repeated after strain loading.21) However, it was revealed by this study, not only yield stress but also tensile strength are improved by strain aging. The improvement of tensile strength due to strain aging is also valid in the case of pressed products that undergo plastic strain from multiaxial directions. In the case of actual products, the time from press processing to product shipment varies, and in some cases, the strain aging must be considered. In particular, it is possible that a difference in strengths of products may arise due to the difference between trial manufacturing (in which a strength test is carried out immediately after forming) and mass production (when there is a certain time from forming to market shipment), and care must be taken in evaluating strength reliability.
3.2 Relation between work hardening and tensile strengthThe results of measuring cross-sectional hardness of the press-worked products in each of the three processes are plotted in Fig. 15. The horizontal axis represents the circumferential length to the end of the product with the central apex of the pressed product as the origin. The results of examining the increase in hardness accompanying press working by using the hardness distribution in Fig. 15 are shown in Fig. 16. In the figure, the measured hardness at the time of tensioning until just before the blank material breaks is shown by the dotted line. It is clear that hardness is increased by each press-working process, and in case of the third-step-pressed product, it is slightly less than twice that of the blank material. Also, the hardness of the third-step-pressed product exceeds the critical hardness under simple tension.
Hardness distribution of press worked product cross section.
Work hardening on tri-axial and uniaxial straining.
The relationship between tensile strength of the pressed product and pre-strained material and average hardness of each test specimen is shown in Fig. 17. It is clear from the figure that the hardness attained when multiaxial strain is applied to the specimens by press working is higher than that attained under uniaxial strain; in other words, the amount of work hardening is larger. In addition, regardless of the histories of uniaxial and multiaxial strains, the hardness obtained by applying processing strain is roughly proportional to tensile strength. It is generally known that material hardness and tensile strength have a proportional relationship, but it became clear from the present study that the same proportionality holds true for work-hardened material. According to this result, if the hardness of a pressed part is known, its tensile strength can be derived. In other words, tensile strength of products subjected to different strains in the manner described above can be uniformly evaluated by hardness measurement.
Relationship between tensile strength and average hardness.
In press forming, the work-piece is subjected to reverse drawing, by which not only tensile stress but also complicated combined multiaxial strain (including compressive stress) is applied to the product. As a result, work hardening is greater and tensile strength is increased more than that obtained with uniaxial strain. Specifically, in the second step of the press-forming process, namely, reverse-press forming, work hardening is performed by applying a stress in the direction opposite to that of the first step. In the third step, compressive stress is applied by intrusion for restoring the thickness of the vertical wall. It is conceivable that unlike applying unidirectional tension, imparting multiaxial distortion can introduce work hardening.
It can therefore be concluded that by effectively utilizing the work hardening of a material during the forming process, it is possible to considerably increase the tensile strength of that material by forming and work hardening complicated product shapes by press forming using a mild-steel plate (which as excellent formability) instead of using a high-strength steel sheet (which faces with many problems, such as formability, shape accuracy, processing load, weld ability, and paint ability). It is expected that effectively utilizing the improvement in tensile strength accompanying the work hardening will provide a weight-saving method enabling stable production with high cost effectiveness.
3.3 Fatigue strength of pressed productsThe results of the fatigue tests on pressed products subjected to multiaxial strain are shown in Fig. 18. When the fatigue strength of the blank material and the third-step-processed product are focused on, it is clear that as well as tensile strength, fatigue strength is improved by press working. Although the results of conventional research have shown that hardness and fatigue strength have a proportional relationship,5,6) it was revealed by the present study that the change in hardness due to work hardening correlates with fatigue strength in the same manner as tensile strength.
Fatigue tests of blank and press worked products.
Differences in work hardening under applied uniaxial strain or multiaxial strain were investigated from the microscopic viewpoint of dislocation density by using the half-width method with X-ray diffraction. X-ray-diffraction peak profiles of the third-step-pressed product and the blank material subjected to uniaxial strain of 20% are compared in Fig. 19(a) and (b), respectively. The ordinate represents diffraction intensity, the abscissa represents diffraction angle. The full width at half maximum of the peak profile (i.e., width of the diffraction angle at half of peak diffraction intensity) is also shown in the figure. Before the strain was applied, the profile shows a sharp rise (labelled “blank”), but when multiaxial and uniaxial strain was imposed, the profile shows a lower diffraction intensity and a wider angular distribution. It is generally known that this tendency is observed when dislocation density increases. Moreover, when the full widths at half maximum in the cases of multi-axis and single-axis strain are compared, it is clear that the full width at half maximum in the case of receiving multiaxial strain is large, so dislocation density must be large too.
Peak profile on X-ray diffraction. (a) pre-straining (press worked products), (b) pre-straining.
The relationship between full width at half maximum of the peak-intensity profile and tensile strength of the three-step pressed product and the uniaxial-pre-strained material is shown in Fig. 20. It is clear from this result that in the same manner as the case of hardness explained in the previous section, not only strain history but also full-width-at-half-maximum intensity and tensile strength are correlated. Dislocation density is higher when multiaxial strain is applied by press working than that produced when uniaxial distortion is imposed. This result demonstrates that it is possible to understand the difference in work hardening from the microscopic point of view.
Relationship between tensile strength and full width at maximum intensity.
In the previous section, dislocation density was investigated by analyzing the peak intensity profile obtained by X-ray diffraction. In this section, the dislocation structures of the third-step-pressed product subjected to multiaxial strain and the uniaxial tension/pre-strained material (plastic strain of 20%) were observed by TEM. As shown in Fig. 21, in both cases of plastic deformation, a “dislocation-cell” texture—due to the entanglement of dislocations proliferated by plastic deformation—is observed. However, differences in that condition can be recognized. That is, in the case of the pressed product, fine grains and equal-axial dislocation cells are observed; however, in the case of uniaxial-tension-loaded specimen, in some regions, the cells are obscure, and the observed cells are coarse and extend in the tensile direction. The shape and size of such cells are closely related to the degree of work hardening, and they are therefore judged to be correlated with the increase in tensile strength that was revealed in this study.
TEM observation of dislocation cell.
In this study, which focused on parts for cars made of mild-steel plate, test specimens were cut out from blank material and pressed products formed in three steps and subjected to tensile tests. The effects of plastic strain applied from multiple-axial directions during each of the pressing processes (three steps) on mechanical properties of the material were investigated. In addition, tensile strength and work-hardening characteristics in the case that strain was applied from multi-axial directions by press processing and in the case that uniaxial strain was applied by tensile test were compared. Furthermore, in a unified manner with the results of the tensile tests, fatigue tests on plate material subjected to multiaxial strain were conducted, and the strength reliability of the pressed product was evaluated. The results of this study are summarized as follows.
